﻿Digital shearography is an interferometric technique for surface deformation
measurement. It was invented to overcome several limitations of holography by
eliminating the reference beam, thus leading to simplified setup, reduced
coherence-length requirement of light source, and not requiring special vibration
isolation. These distinct advantages have rendered shearography as a practical and
employable measurement tool in industrial settings. In fact, it has already gained
widely industrial acceptance, in particular for nondestructive testing applications.
Shearography is particularly effective for detecting debonds in laminated composite
materials. Hence, the rubber industry has been routinely employing shearography for
evaluating tire quality, and the technique has been endorsed by the US Federal
Aviation Administration (FAA) for inspecting aircraft tires. Shearography, however,
is still relatively young and its full capability awaits further exploration. This research
has developed several new techniques of shearography for dynamic deformation
measurement and nondestructive testing.
A new form of shearography referred to as Spatial-Frequency-Modulated (SFM)
shearography has been developed. Traditionally, the output of shearography is in the
form of a fringe pattern depicting the phase change due to deformation. To determine
the phase change, it is necessary to capture at least three speckle images with different
values of phase shifting at each deformed state. The requirement of phase shifting
presents a problem in measuring dynamic deformation. The SFM shearography
requires acquisition of only one single speckle image for each deformed state, thus
enabling dynamic deformation measurement. In SFM shearography, a modulation fringe pattern in the form of a linear and parallel fringe lines is initially introduced
before deformation. The production of the modulation fringe pattern is achieved by
introducing a quadratic phase variation in the illumination laser light. Since
shearography measures the first derivative of the phase change and the first derivative
of a quadratic phase variation is a linear phase distribution, a fringe pattern of uniform
spatial frequency (i.e. linear fringe lines of equal spacing) is produced. When the
object is deformed, the SFM pattern will be distorted, thus resulting in a variation of
spatial frequency change in the modulation fringe pattern, and the local frequency
change carries the information about the deformation. In essence, a linear phase
distribution is superimposed on the deformation phase of traditional shearography. To
extract the deformation phase, an algorithm based on the Moiré effect is developed.
A Moiré fringe pattern is generated by the multiplication of the deformed SFM pattern
and a virtual linear grating digitally simulated in the computer memory. Since the
phase of Moiré fringes can be changed by shifting the virtual grating, a conventional
phase-shifted algorithm, such as the four-frame algorithm, can be used to determine
the deformation phase. This process is carried out subsequent to a dynamic
recording. The validity of the SFM shearography has been verified by measuring a
plate structure of known deformation.
In NDT applications, shearography reveals a material defect by identifying
defect-induced deformation. Coupling with the SFM shearography, two new methods
of nondestructive testing using dynamic stressing have been developed. Flash
shockwave method irradiates an intense energy of very short duration to the test
object surface thus inducing a transient thermal deformation. Induction method
heats the electrically conducting test material by electromagnetic induction theory.
Both methods do not produce intolerable rigid body motion nor change the reflective indexof surrounding air which can cause de-correlation in the speckle images. The
dynamic deformation produced by both methods may be measured by SFM shearography.
Another significant research contribution addresses a tall-building safety problem
in Hong Kong. One of the main emphases in high-rise building maintenance is to
locate areas where wall tiles are prone to detaching from external building walls, since
fallen tiles endanger public safety. The research has developed sonic-shearography
for testing the integrity of bonding between tiles and building walls. In this technique,
the test area is excited using broadband frequencies generated by a
computer-controlled powerful loudspeaker. The shearographic image of the test area
is recorded continuously, and the recorded images are compared with one another.
Since each pixel is corresponding to a point on the tiled wall surface, the algorithm
developed to determine the peak deformation during the excitation allows the
deboned area to be identified.